Sound-insulating laminated structure and method for the production thereof

ABSTRACT

Herein provided is a sound-insulating laminated structure which is excellent in the ability of insulating low frequency sounds whose frequency falls within the range of from 200 to 1000 Hz. The sound-insulating laminated structure comprises a first glass wool layer, a metal foil layer free of any opening and a second glass wool layer. The first and second glass wool layers are ones each prepared by collecting and combining short glass fibers through the application of an uncured thermosetting phenol resin to the short glass fibers to thus give a web of glass wool prepreg and then molding, with heating, the web into a sound-insulating laminated structure having a desired shape.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/521,203, filed Jun. 25, 2009, which was the National Stage filingunder §371 of PCT/JP2007/072271, filed Nov. 16, 2007, which claimspriority to Japanese Patent Application No. 2007-007658, filed Jan. 17,2007, the entire content of each of which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a sound-insulating laminated structurehaving an excellent ability of insulating or absorbing low frequencysounds.

TECHNICAL BACKGROUND ART

There has conventionally been used glass wool, for instance, around theengines of automobiles in order to insulate noises from the engines orthe like. Such glass wool used therein has been produced by applying anuncured thermosetting resin to spun short glass fibers, collecting andcombining the short glass fibers to thus give a web and thenpressure-molding the resulting web with heating. The temperature isconsiderably increased around, for instance, an engine during theoperation thereof and therefore, various members used in thesurroundings of the engine should be heat-resistant so that it canendure the temperature on the order of not less than 200° C. or even notless than 250° C. For this reason, phenolic resins have been used as theforegoing thermosetting resins (see Patent Document 1 specified below).

Recently, the so-called hybrid type automobiles have come intowidespread use, which are ones each comprising a combination of agasoline or diesel engine and an electric motor. The automobile equippedwith an electric motor is quite excellent in the silentness andaccordingly, the low frequency sounds, generated by the electric motor,on the order of not higher than 1000 Hz and more specifically thoseranging from 200 to 1000 Hz have recently been recognized as noises.However, it is difficult that the aforementioned glass wool formed bypressure-molding with heating would insulate the sounds whose frequencyfalls within the range specified above and accordingly, there has beendesired for the development of a novel sound-insulating material.

Patent Document 2 specified below discloses, as a structure forinsulating the foregoing low frequency sounds, a sound-insulatingmaterial having such a structure that a fibrous material such as glasswool is laminated with a nonwoven fabric consisting of, for instance,polyethylene terephthalate serving as an intermediate layer. This PatentDocument 2 discloses that the sound insulation property of the structurewith respect to the foregoing low frequency sounds can be improved bymaking the air-permeability of the nonwoven fabric lower.

In addition, there has likewise been disclosed a sound-insulatingstructure in which layers of a fibrous material such as glass woolcomprise a layer of a different material inserted therein as anintermediate layer. For instance, Patent Document 3 specified below usesan iron plate as such an intermediate layer and Patent Document 4 uses aresin film in which fine pores are uniformly formed in such a mannerthat the area of the film occupied by the fine pores (rate of pores)falls within the range of from 0.05 to 5%.

-   Patent Document 1: Japanese Un-Examined Patent Publication    2001-337681;-   Patent Document 2: Japanese Un-Examined Patent Publication    2003-19930;-   Patent Document 3: Japanese Un-Examined Utility Model Publication    Sho 61-11446; and-   Patent Document 4: Japanese Un-Examined Patent Publication    2006-137160.

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

Although the sound insulation property of a structure can be improved byincreasing the thickness of the structure, the permissible increase ofthe thickness thereof is limited in a spatially restricted place such asthe place around a vehicle engine. Moreover, the structure should beformed into any desired shape depending on the kind of materials to beused, which in turn limits the kinds of materials used for forming thestructure. In addition, the structure is used as a part surrounding thevehicle engine, it should be heat-resistant so that it can endure thetemperature on the order of not less than 200° C. Furthermore, thestructure should maintain its shape and construction even under theinfluence of any vibration generated due to the operation of the engine.

Accordingly, it is an object of the present invention to provide astructure excellent in the ability to insulate low frequency soundswhose frequency falls within the range of from 200 to 1000 Hz, whiletaking into consideration the foregoing restrictions encountered whenselecting materials for forming the structure and the limitation in thethickness. For instance, the overall thickness of the structure isherein limited to a level falling within the range of from 5 to 100 mm,preferably 10 to 70 mm, and more preferably 15 to 40 mm.

Means for the Solution of the Problems

The present invention aims to provide a structure which has good heatresistance so that it can be used around an engine and which may haveany desired shape. Glass wool may easily provide an article having anydesired shape by appropriately selecting the shape of a mold whenmolding the same with heating. In addition, to impart the foregoing lowfrequency sound-insulating property to the structure, the material forthe structure should be excellent in the ability of insulating soundswhose frequency falls within the range specified above and it should beable to be formed into a desired shape. Accordingly, the structureshould be formed from a material excellent in the sound-insulatingproperty and heat resistance and likewise excellent in the flexibility.

The transmission loss encountered when sounds transmit through alaminated structure is greatly influenced by the magnitude of therigidity of the material used for forming an intermediate layer of thelaminated structure. If the rigidity of the material for theintermediate layer is relatively low, the transmission loss would beable to enhance the ability of insulating the sounds passing though thestructure while making the most use of the resonance effect observedbetween the intermediate layer and the materials positioning at bothsides thereof. Further, it has been said that when intending to use sucha resonance effect, the use of a porous material is required so that theintermediate layer may almost show the same effect achieved by a layerof air.

However, even a porous material has an elastic modulus higher than thatof the air and accordingly, to achieve a sound-insulating effectapproximately identical to that observed for the air layer, thethickness of the intermediate layer must be increased or theintermediate layer must be so designed that it has a high porosityalthough there would be a risk of impairing the strength of theresulting structure. However, it is difficult to adopt such a design inthe present invention since the invention intends to provide a structurehaving such a limited thickness.

On the other hand, if increasing the rigidity of a material used forforming such an intermediate layer, the transmission loss of the soundspassing through a structure would be almost controlled or approximatedby the mass rule and therefore, the mass and thickness of a materialused should be increased when intending to enlarge the transmission lossthereof. However, it is likewise difficult in the present invention toselect such a material since the invention aims to produce a structurehaving such a limited thickness.

Under the foregoing hard conditions, the inventor of this invention hasconducted various investigations of various materials to be used andstructures to develop a structure which can accomplish the foregoingobject and have found that the use of a metal foil layer as anintermediate layer for such a structure would permit the improvement ofthe ability of insulating low frequency sounds whose frequency rangesfrom 200 to 1000 Hz.

More specifically, the sound-insulating laminated structure according tothe present invention is one comprising a first glass wool layer, ametal foil layer free of any opening and a second glass wool layer,wherein the first and second glass wool layers have been each preparedby applying an uncured thermosetting phenol resin onto short glassfibers thereof and then collecting and combining the short glass fibersto thus give a web and then molding, with heating, the web into adesired shape.

In the meantime, the metal foil having openings, which has been used asthe intermediate layer for the conventional sound-insulating laminatedstructure, is composed of a metal and therefore, the elastic modulus ofthe material per se should be considerably greater than that observedfor the layer of air used in the structure comprising such a layer ofair and an intermediate layer. Contrary to this, the inventor of thisinvention has unexpectedly found that the use of a metal foil free ofany opening as an intermediate layer would permit the improvement of theability of insulating low frequency sounds whose frequency falls withinthe range of from 200 to 1000 Hz, and especially sound waves having afrequency ranging from 300 to 900 Hz and, in particular, sound waveshaving a frequency ranging from 400 to 900 Hz, and further low frequencysound waves having a frequency ranging from 400 to 700 Hz.

Although not being bound by any specific theory, it would be presumed asfollows: In the structure comprising a metal foil as an intermediatelayer, the metal foil shows flexibility to some extent, the flexibilitythereof may in turn induce the resonance effect between the soundstransmitting through the structure: “first glass wool layer/metal foillayer free of any opening/second glass wool layer” and the soundsreflected from the same and as a result, the structure is thus improvedin the sound-insulating property.

The foregoing metal foil layer free of any opening is preferablyintegrated, into a body, with the both glass wool layers through theaction of the uncured phenol resin incorporated into the web of theshort glass fibers. If the structure has such a construction, the metalfoil layer free of any opening and the both glass wool layers can beunited in a body without using any other material and accordingly, itwould be quite easy for the structure to show its resonance effect dueto the presence of the metal foil layer.

In the present invention, the overall thickness of the structure is setat a level ranging from 5 to 100 mm, preferably 10 to 70 mm, and morepreferably 15 to 40 mm, while the metal foil layer free of any openingis arranged at a position situating at a distance, from the outermostface of the structure, of 0.1 to 0.5 times, preferably 0.15 to 0.45times, more preferably 0.2 to 0.4 times and more particularly preferably0.3 to 0.4 times the thickness of the structure, in the direction of thethickness thereof, in order to achieve a higher sound-insulating effectwhile the thickness of the structure is limited to such a low levelspecified above.

Further, the structure according to the present invention, whichcomprises a first glass wool layer, a metal foil layer free of anyopening and a second glass wool layer, may further comprise a metal foillayer having openings which may be put on top of the first or secondglass wool layer. The structure of the present invention having theforegoing construction has an improved ability of insulating lowfrequency sounds having a frequency ranging from 200 to 1000 Hz, whilesimultaneously reducing the ability of insulating middle frequencysounds having a frequency ranging from 1000 to 2000 Hz. The use of sucha metal foil having openings is quite desirable for the improvement ofthe ability of insulating sounds having a frequency falling within therange specified above. In this respect, if the structure is alsoprovided with a metal foil layer as an intermediate layer, thisaccordingly results in the possible interactions between the soundspassing through the metal foil layer having openings and the metal foillayer free of any opening to thus improve the effect of insulating lowfrequency sounds having a frequency ranging from 200 to 1000 Hz.

More specifically, the laminated structure of the present inventionpreferably comprises a first glass wool layer, a metal foil layer freeof any opening, a second glass wool layer, a metal foil layer havingopenings and a third glass wool layer, which are laminated in thisorder; or a third glass wool layer, a metal foil layer having openings,a first glass wool layer, a metal foil layer free of any opening and asecond glass wool layer, which are likewise laminated in this order. Thestructure having such a construction is preferred since it is quitesuitable for causing interactions between the sounds passing through themetal foil layer having openings and the metal foil layer free of anyopening. In this respect, when the structure comprises one each of themetal foil layer free of any opening and the metal foil layer havingopenings, the resulting structure of the present invention includesthree glass wool layers.

The metal foil layer having openings is preferably integrated, into abody, with the both glass wool layers existing at both sides thereofthrough the action of the uncured phenol resin incorporated into the webof the short glass fibers constituting the glass wool layers. Thestructure of the present invention having such a construction may thuseasily show its resonance effect observed between the metal foil layerfree of any opening and the metal foil layer having openings.

In the present invention, the “metal foil layer free of any opening”means a metal foil free of any hole passing from the surface to the backface of the foil (through hole) and therefore, any gas cannot pass orpermeate through the metal foil. On the other hand, the “metal foillayer having openings” means a metal foil provided with through holes,through which a gas can pass.

In addition, the (imaginary) center line of the metal foil layer havingopenings is arranged at a position situating at a distance, from theoutermost face of the structure or the face thereof exposed to the air,of 0.1 to 0.5 times, preferably 0.15 to 0.45 times, more preferably 0.2to 0.4 times and more particularly preferably 0.3 to 0.4 times thethickness of the structure, in the direction of the thickness thereof,in order to achieve a higher sound-insulating effect while the thicknessof the structure is limited to such a level specified above. Theoutermost face of the structure, which can serve as the reference fromthe viewpoint of achieving a higher sound-insulating effect while usinga limited thickness of the structure, is preferably one arrangedopposite to the outermost face of the structure which is used or servesas the reference for arranging the metal foil layer free of any opening.

The metal foil layer free of any opening or the metal foil layer havingopenings has in general a thickness ranging from 20 to 150 μm,preferably 40 to 120 μm, and more preferably 50 to 90 μm. This isbecause, if the thickness thereof exceeds 150 μm, it is quite difficultto have the flexibility of the metal foil, while if it is less than 20μm, it is difficult to handle such a metal foil and the productionefficiency of the structure may sometimes be reduced. In addition, ifthe thickness of the metal foil is too small, the used structures can bebroken down and separated into the metal portions and the glass woolportions only with great difficulty after the recovery thereof from theconsumers. This accordingly greatly reduces or impairs the recyclingability of the structure.

The sound-insulating laminated structure of the present invention caneasily be prepared by collecting and combining short glass fibers afteradhering the short glass fibers to one another through the applicationof an uncured thermosetting phenol resin to the same to thus give a webof glass wool prepreg, then sandwiching a metal foil between two layersof the glass wool prepreg to thus give a laminate structure andsubjecting the laminate structure to pressure-molding with heating.

Effects of the Invention

The structure of the present invention is formed from a glass wool layerand a metal foil and therefore, the structure can provide asound-insulating laminated structure excellent in the heat resistance,and likewise excellent in the ability of insulating low frequency soundshaving a frequency ranging from 200 to 1000 Hz without unreasonablyincreasing the thickness thereof. Accordingly, the structure of thepresent invention can effectively be used as not only thesound-insulating material for the construction such as houses andbuildings, but also as the sound-insulating material used around, forinstance, the motors and engines of compressors, railway vehicles andautomobiles, in particular, around the motors and engines ofautomobiles.

BEST MODE FOR CARRYING OUT THE INVENTION

The sound-insulating laminated structure of the present inventioncomprises a first glass wool layer, a metal foil layer free of anyopening and a second glass wool layer, wherein the first and secondglass wool layers are glass wool layers formed by collecting andcombining short glass fibers after adhering the short glass fibersthrough the application of an uncured thermosetting phenol resin theretoto thus give a web of glass wool prepreg and then subjecting the prepregweb to molding with heating.

The glass wool is a material widely used as a fibrous sound-insulatingand heat-insulating material and it is typically produced according tothe following method:

An uncured thermosetting phenol resin is adhered to short glass fiberswhich are spun according to the centrifugal spinning technique and havean average diameter ranging from 3 to 20 μm, preferably 5 to 15 μm, andan average length ranging from 1 to 100 mm and preferably 3 to 80 mm, byspraying the same through, for instance, a spray nozzle to thus form aglass wool prepreg. The volume density of the glass wool prepreg ispreferably set at a value ranging from, for instance, 5 to 50 kg/m³,preferably 8 to 20 kg/m³. In this respect, the use of, for instance, amold for processing a material into an article having a desired shapewould permit the pressure-molding, with heating, the web-like glass woolprepreg and the control of the thickness thereof and this results in theformation of a glass wool layer having a volume density ranging from 10to 500 kg/m³, and preferably 15 to 300 kg/m³.

The foregoing pressure-molding is preferably carried out under thefollowing conditions: a temperature, for instance, ranging from 150 to350° C.; a pressure, for instance, ranging from 2×10⁴ to 2×10⁶ Pa,preferably 2×10⁵ to 2×10⁶ Pa; and a molding time, for instance, rangingfrom 10 to 3600 seconds and preferably 20 to 600 seconds.

In the structure according to the present invention, the volumedensities of the glass wool layers such as the first glass wool layerand the second glass wool layer and even the third glass wool layer maybe set at the same level or the volume densities of the foregoing glasswool layers may be different from each other. When the volume densitiesof the glass wool layers are set at different values respectively, thevolume density of each glass wool layer is so controlled that the glasswool layer has a volume density of not higher than 30 times, preferablynot higher than 20 times, more preferably not higher than 10 times andparticularly preferably not higher than 5 times that of the glass woollayer having the smallest volume density, which is defined to be areference.

The smallest volume density of the glass wool layer in general rangesfrom 10 to 200 kg/m³, and preferably 15 to 500 kg/m³.

In this connection, the average diameter of the fibers and the densityof articles used in the present invention are determined according tothe method specified in “JIS A 9504 (2004); “Heat-Insulating Materialsof Artificially Prepared Mineral Fibers.” Moreover, the average lengthof fibers used in the present invention is an average value obtained bydetermining the length of the fibers extracted when determining theaverage diameter thereof and then dividing the sum of the values thusdetermined by the number of the measurements and specifically samples(20 g each) are collected from an article at arbitrarily selected 3positions thereon, fibers (20 fibers each) are further extracted fromthe foregoing samples and then the sum of the lengths of these fibers isdivided by the total number of measurements to thus calculate an averagelength of fibers.

The amount of the uncured phenol resin to be adhered to the short glassfibers is preferably so controlled that the volume density of theresulting glass wool falls within the range specified above and thespecific amount of the phenol resin ranges from 0.05 to 0.2 times thetotal amount of the short glass fibers.

The phenol resin used herein is, for instance, one obtained bycondensing a phenolic compound such as phenol, xylenol or cresol with analdehyde such as formaldehyde or paraformaldehyde at a mixing ratioranging from 1.0:0.5 to 1.0:0.9, preferably in the presence of acatalyst such as an acid catalyst selected from oxalic acid and zincacetate or a catalyst consisting of a divalent metal salt. The phenolresin forms a three dimensional network structure after the resin iscured. Among the thermosetting synthetic resins, the phenol resin isparticularly excellent in the heat resistance and the flame-retardantproperties and likewise excellent in the resistance to oils andresistance to chemicals and accordingly, it is quite suitably used inthe structure of the present invention.

In the present invention, a variety of binders currently used in theproduction of glass wool and rock wool can be used without anyparticular restriction as the phenol resins and, in particular,water-soluble phenol resins are suitably used in the present invention.Examples of such phenol resins usable herein include resol-type phenolresins and urea-modified resol-type phenol resins.

The metal foil used as the metal foil layer free of any opening and themetal foil layer having openings in the structure according to thepresent invention is one prepared from a metal which can easily beformed into a foil and which has a melting point of not less than 200°C., preferably not less than 250° C. and more preferably not more than2000° C. Examples of such metals preferably used herein includealuminum, silver, gold, copper, nickel, iron and stainless steel. Amongthem, preferred is aluminum. Examples of aluminum preferably used hereininclude pure aluminum products, for instance, Aluminum of 1000 Series asspecified in JIS 114000 “Plates and Wires Made of Aluminum and AluminumAlloys (2006)” such as A11050, A11070, A11085, A11100, and A11200; andaluminum alloys having relatively good processability, for instance,Al—Mn type alloys called Aluminum of 3000 Series such as A13003, A13004and A13104.

In case of the metal foil having openings, it is preferred to use metalfoils each having a rate of openings in general ranging from 10 to 80%,preferably 15 to 70% and further preferably 20 to 60%. In addition, thesize of each opening may be set at a level on the order of 20 to 1000mm², preferably 50 to 600 mm², and more preferably 80 to 500 mm², andthe shapes thereof usable herein include, for instance, circular,rectangular, and polygonal ones.

The laminated structure of the present invention can preferably beprepared by sandwiching a metal foil free of any opening between twolayers of glass wool prepreg and then subjecting the resulting assemblyto pressure molding with heating. Regarding this superposed structureprior to the molding, the structure of the present invention mayalternatively be prepared by sandwiching each of a metal foil free ofany opening and a metal foil having openings between correspondinglayers of glass wool prepreg so that the resulting superposed structurehas the construction defined above and then subjecting the superposedstructure to pressure-molding with heating.

Upon the pressure-molding with heating, the uncured phenol resin adheredto the short glass fibers can penetrate into the glass wool layer andeven to the metal foil layers to thus unite the metal foil layers andthe glass wool layers in a body. The region of the glass wool layerswhich are impregnated with the phenol resin may appropriately becontrolled by adjusting the pressure, temperature and time of thepressure-molding operation.

FIG. 1 is a diagram given for explaining the processes for forming thestructure according to the present invention. FIG. 2 is a diagramshowing an embodiment of the sound-insulating laminated structure of thepresent invention.

More specifically, a laminated structure is produced by sandwiching ametal foil 21, which will later serve as a metal foil layer 2 free ofany opening, between two layers of glass wool prepreg 3 to which anuncured phenol resin has been applied; and then subjecting the resultinglaminated structure to pressure-molding with heating to thus produce asound-insulating laminated structure 4 which comprises a first glasswool layer 1, a metal foil layer 2, and a second glass wool layer 1arranged and united, in a body, in this order from the top.

The sound-insulating laminated structure of the present invention can bemolded into a structure having any desired shape if appropriatelyselecting the shape of the mold used in the step for pressure-molding ofthe same. FIG. 2 shows an embodiment of the structure of the presentinvention and the structure having the construction as depicted in thisfigure can serve as a sound-insulating laminated structure of thepresent invention excellent in the sound-insulating property and havinga bent portion A.

The present invention will hereunder be described in more detail withreference to the following Examples and Comparative Examples.

EXAMPLE

Short glass fibers each having an average diameter of 7 μm and anaverage length of 30 mm were prepared by the centrifugal spinningtechnique and an uncured water-soluble phenol resin (resol type onehaving a solid content of 30% by mass) was adhered to the spun shortglass fibers by spraying the same on the fibers in an amount of 0.1 time(by mass) that of the short glass fibers using a spray gun. The shortglass fibers provided with the uncured phenol resin adhered thereto werefed to a fiber-collector in which the fibers were integrated throughaspiration to thus give a glass wool prepreg having a volume density of16 kg/m³.

On the other hand, an aluminum metal foil having a predeterminedthickness (20 μm, 50 μm and 85 μm) was sandwiched between two layers ofthe foregoing glass wool prepreg to form a superposed structure and thenthe superposed structure was heated to 250° C. for 200 seconds whileapplying a pressure of 2×10⁵ Pa to thus give a laminated structurehaving an overall thickness of 30 mm and the following construction:glass wool layer/metal foil layer free of any opening/glass wool layer.

In the meantime, the volume densities of the two glass wool layers usedin this Example are identical to one another since these two layers areprepared from the materials or the glass wool prepregs having the samevolume density. Regarding the glass wool layer, laminated structuresprovided with glass wool layers having a variety of volume densities canbe prepared by variously controlling the thickness of the glass woolprepreg used. Thus, the position of the metal foil layer free of anyopening in the resulting sound-insulating laminated structure can bedetermined depending on the difference in the volume density of theglass wool layers to be used.

Summarized in the following Table 1 are the volume density of the glasswool layer of each resulting sound-insulating laminated structureprepared in this Example and the position of the metal foil layer ineach resulting sound-insulating laminated structure along the totalthickness thereof. The position of each metal foil layer can bedetermined by dividing the distance extending from the outermost face ofeach sound-insulating laminated structure to the corresponding metalfoil layer by the total thickness of the sound-insulating laminatedstructure in question. In Table 1, the abbreviation “GW” means the glasswool. In Table 1, “Not Used” means that the corresponding comparativesample is given by way of comparison and is free of any metal foil, or astructure which does not comprise any metal foil layer, or a structurecomprising a single glass wool layer. Moreover, the numerical valueappearing in each sample name after the hyphen (-) represents thethickness of the metal foil layer used in each sample structure.

TABLE 1 Volume Density of Position Thickness of Metal Sample Name GW(kg/m³) of M.F.L.* Foil (μm) Sample 1-20 33.3 0.5 20 Sample 1-50 33.30.5 50 Sample 1-85 33.3 0.5 85 Sample 1-0 33.3 — Not Used Sample 2-20 500.333 20 Sample 2-50 50 0.333 50 Sample 2-85 50 0.333 85 Sample 2-0 50 —Not Used Sample 3-20 66.6 0.25 20 Sample 3-50 66.6 0.25 50 Sample 3-8566.6 0.25 85 Sample 3-0 66.6 — Not Used Sample 4-20 83.3 0.2 20 Sample4-50 83.3 0.2 50 Sample 4-85 83.3 0.2 85 Sample 4-0 83.3 — Not UsedSample 5-20 100.3 0.167 20 Sample 5-50 100.3 0.167 50 Sample 5-85 100.30.167 85 Sample 5-0 100.3 — Not Used *M.F.L. means the metal foil layerfree of any opening.

[Evaluation of the Sound-Insulating Property of Example]

The rates of sound-insulation of the sound-insulating laminatedstructures as the sample Nos. 1 to 5 with respect to the soundsperpendicularly incident upon the samples were determined according tothe method specified in the appendix B of JIS A6301 “Sound-InsulatingMaterials (2000).” In this evaluation method, the thinner glass woollayer was positioned on the side of the sound source.

The results thus obtained are plotted on the attached FIGS. 3 to 7. Aswill be seen from these figures, it was found that the sound-insulatinglaminated structure of the present invention was excellent in the lowfrequency sound-insulating ability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram for explaining the processes of producingthe sound-insulating laminated structure of the present invention.

FIG. 2 is a diagram showing an embodiment of the sound-insulatinglaminated structure of the present invention.

FIG. 3 is a diagram showing the results obtained when the sample 1prepared in the foregoing Example of the present invention was inspectedfor the sound-insulating ability.

FIG. 4 is a diagram showing the results obtained when the sample 2prepared in the foregoing Example of the present invention was inspectedfor the sound-insulating ability.

FIG. 5 is a diagram showing the results obtained when the sample 3prepared in the foregoing Example of the present invention was inspectedfor the sound-insulating ability.

FIG. 6 is a diagram showing the results obtained when the sample 4prepared in the foregoing Example of the present invention was inspectedfor the sound-insulating ability.

FIG. 7 is a diagram showing the results obtained when the sample 5prepared in the foregoing Example of the present invention was inspectedfor the sound-insulating ability.

EXPLANATION OF THE LETTERS OR NUMERALS

-   1 . . . Glass Wool Layer-   2 . . . Metal Foil Layer Free of Any Opening-   3 . . . Glass Wool Prepreg-   4 . . . Sound-Insulating Laminated Structure-   21 . . . Metal Foil-   . . . A Bent Portion.

1. A method for reducing low frequency sound generated by an electricmotor or engine of automobile, the method comprising placing asound-insulating laminated structure around the electric motor orengine, wherein the sound-insulating laminated structure comprises afirst glass wool layer, a metal foil layer free of any opening and asecond glass wool layer, wherein the first and second glass wool layersare ones each prepared by collecting and combining short glass fibersthrough the application of an uncured thermosetting phenol resin to theglass fibers to thus give a web of glass wool prepreg and then molding,with heating, the web into a desired shape, wherein the overallthickness of the structure ranges from 5 to 100 mm, and wherein the lowfrequency sound ranges from 200 to 1000 Hz.
 2. The method as set forthin claim 1, wherein the overall thickness of the structure ranges from10 to 70 mm.
 3. The method as set forth in claim 1, wherein the overallthickness of the structure ranges from 15 to 40 mm.
 4. The method as setforth in claim 1, wherein the low frequency sound ranges from 300 to 900Hz.
 5. The method as set forth in claim 1, wherein the low frequencysound ranges from 400 to 900 Hz.
 6. The method as set forth in claim 1,wherein the low frequency sound ranges from 400 to 700 Hz.
 7. The methodas set forth in claim 1, wherein the metal foil layer free of anyopening is arranged at a position situating at a distance, from theoutermost face of the structure, of 0.1 to 0.5 times the thickness ofthe structure, in the direction of the thickness thereof.
 8. The methodas set forth in claim 1, wherein the metal foil layer free of anyopening is arranged at a position situating at a distance, from theoutermost face of the structure, of 0.15 to 0.45 times the thickness ofthe structure, in the direction of the thickness thereof.
 9. The methodas set forth in claim 1, wherein the metal foil layer free of anyopening is arranged at a position situating at a distance, from theoutermost face of the structure, of 0.2 to 0.4 times the thickness ofthe structure, in the direction of the thickness thereof.
 10. The methodas set forth in claim 1, wherein the metal foil layer free of anyopening is arranged at a position situating at a distance, from theoutermost face of the structure, of 0.3 to 0.4 times the thickness ofthe structure, in the direction of the thickness thereof.
 11. The methodas set forth in claim 1, wherein the metal foil layer has a thicknessranging from 20 to 150 μm.
 12. The method as set forth in claim 1,wherein the metal foil layer has a thickness ranging from 40 to 120 μm.13. The method as set forth in claim 1, wherein the metal foil layer hasa thickness ranging from 50 to 90 μm.
 14. The method as set forth inclaim 1, wherein the sound-insulating laminated structure is prepared bythe steps of: (1) providing a web of glass wool prepreg by collectingand combining short glass fibers through the application of an uncuredthermosetting phenol resin to the short glass fibers; (2) sandwiching ametal foil free of any opening between two layers of the foregoing webto thus form a laminated structure having a three-layer structure; and(3) pressure-molding the laminated structure prepared in the step (2)with heating.